Multi-zone completions present conflicting design parameters that need to be considered. The need for versatility in being able to shut off individual screens has to be balanced with the resistance to flow that comes from additional tubulars associated with individual screens and the difficulty in channeling flow from one screen through small passages external to the central production flow path. Addition of tubular sections to create such communication paths between screens can also impede subsequent production by reducing the drift diameter available for such flow. The reduced drift dimension also makes access further downhole more complex or impossible in some cases.
FIG. 1-3 illustrate one such system. In FIG. 1 a port 10 on a sliding sleeve valve 12 is in the open position for taking gravel pack returns through screen 14. Arrows 16 represent the gravel slurry flow to the annular space outside screens 14, 20 and 22. Arrows 18 represent the gravel slurry return flow to the surface with the gravel 24 deposited outside screens 14, 20 and 22 as shown in FIG. 2. Because sliding sleeve valve 26 is closed all the return flow represented by arrow 18 is through ports 10 while connecting paths between screens such as 28 are closed. When it is time for production valves 12 and 26 are both opened as shown in FIG. 3. However, the path of least resistance is represented by arrows 30 and 32 as opposed to connecting paths such as 28. As a result, high velocities occur through the screens such as 14 and 22 causing erosion that can put a hole in such screens and further creating uneven flow from the formation as little flow gets to the surface through screen 20 that represents a path of higher resistance than flow coming in radially through screen 14 or 22. Apart from the above issues the presence of sleeves such as 30 to create the network of connecting passages 28 results in a smaller drift dimension for production flow and can reduce the production rate. Apart from this the assembly of such a string is fairly complex and presents many leak path opportunities. It fails to accomplish an objective of maximizing production in an even manner across the screens of the entire zone and is further expensive to construct and install into the borehole.
FIGS. 4-6 represent another prior system where perforations 32, 34 and 36 are sequentially made with a perforating gun that is not shown and the completion is assembled in the hole in stages as shown in FIGS. 4-6. Screen assemblies 36 and 38 are the same and they comprise a screen 40 covered internally by an inner pipe 42 to create an annular space 44 that is accessible by a sliding sleeve valve 46 to take returns when gravel packing. Valve 46 can then be closed when the screen that is associated with it is fully gravel packed. Further access to the annular space 44 is provided by pressure sensitive valves such as 48. These can operate in response to a pressure of a predetermined value that arms each of the similarly situated valves such as 50 and 52 such that when the pressure is released all the valves will open at the same time. The production can then ensue through screen assemblies 36 and 38 into the production string 54 that extends through production packer 56. Flow through screen assembly 58 passes through packer 60 and production packer 56 to come up around the production string 54 as represented by arrows 62. Arrows 64 represent the flow coming through screen assemblies 36 and 38 into the production string 54. The issue with this system is that once the zones are opened with valves 48, 50 and 52 the screens either cannot be closed off, or it is not economical to do so. The same valves can be inadvertently open with a pressure spike in the bottom hole assembly. The use of discrete inner tubulars such as 42 inside each screen also reduce drift diameter that can restrict production.
Also relevant to such systems are U.S. Pat. Nos. 8,511,380; 8,225,863; 6,530,431; 2004/0251033; U.S. Pat. Nos. 8,281,854; 6,776,241 and 20150034301.
The present invention seeks to overcome the shortcomings of the prior systems by using shrouds to span over a base pipe joint so as to create a continuous annular space between the screen assemblies and the base pipe so that the formation can flow more uniformly through a larger drift base pipe because the screens are coupled together. In this manner, a single valve can control return flow when gravel packing. Another valve communicating with the same exterior annular space can be used as the production valve drawing the fluid that is produced through all the screens in the screen assembly. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawings while recognizing that the full scope of the invention is to be determined from the appended claims.